Conventionally, a control pattern has been known in which, in a spark ignition internal combustion engine, a mechanical compression ratio is set to be an extremely great value (e.g., 20 or more), and a closing timing of an intake valve is considerably retarded from the intake bottom dead center (e.g., is retarded by 100° CA from the intake bottom dead center), in order to control the expansion ratio to be an extremely great value (e.g., 20 or more), while keeping the actual compression ration to be an appropriate range (e.g., 8 to 9) (e.g., see Japanese Unexamined Patent Application No. 2007-303423).
This control pattern is called Atkinson cycle. As the mechanical compression ration increases, the expansion ratio increases. As the expansion ratio increases, the period when a pushing-down force is applied to a piston during an expansion stroke increases, so that a thermal efficiency is enhanced. Accordingly, in the Atkinson cycle, the thermal efficiency (accordingly, fuel efficiency) of the internal combustion engine can be enhanced, while securing an appropriate combustion (specifically, without causing a knocking caused by an excessive actual compression ratio or misfire caused by very small actual compression ratio).